Doubtful. If Germany had let up on them, and allowed say, 10 years of peace for Britain to ramp up, then who knows. But they didn't. Germany was driving on them relentlessly and would have swamped them.

Possibly, since 'by itself' meant not just Britain in those days, but the British Empire as well, which was a lot of people, the population of India, Canada, Australia, New Zealand, and large chunks of African and the West Indies. It would have been much more difficult though and taken much longer.